1. Technical Field
The present disclosure relates generally to radio frequency (RF) circuitry, and more particularly, to power amplifier architectures with input power protection circuits.
2. Related Art
Wireless communications systems find applications in numerous contexts involving information transfer over long and short distances alike, and there exists a wide range of modalities suited to meet the particular needs of each. Chief amongst these systems with respect to popularity and deployment is the mobile or cellular phone, and it has been estimated that there are over 4.6 billion subscriptions worldwide.
Generally, wireless communications involve a radio frequency (RF) carrier signal that is variously modulated to represent data, and the modulation, transmission, receipt, and demodulation of the signal conform to a set of standards for coordination of the same. Many different mobile communication technologies or air interfaces exist, including GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and UMTS (Universal Mobile Telecommunications System). Various generations of these technologies exist and are deployed in phases, with one common third generation (3G) UMTS-related modality referred to as UMTS-FDD (frequency division duplexing) being W-CDMA (Wideband Code Division Multiplexing). More recently, 4G (fourth generation) technologies such as LTE (Long Term Evolution), which is based on the earlier GSM and UMTS standards, are being deployed. Besides mobile communications modalities such as these, various communications devices incorporate local area data networking modalities such as Wireless LAN (WLAN)/WiFi, ZigBee, and so forth. Along these lines, last-mile wireless broadband access technologies such as WiMAX (Worldwide Interoperability for Microwave Access) are also being implemented.
A fundamental component of any wireless communications system is the transceiver, that is, the combined transmitter and receiver circuitry. The transceiver encodes the data to a baseband signal and modules it with an RF carrier signal. Upon receipt, the transceiver down-converts the RF signal, demodulates the baseband signal, and decodes the data represented by the baseband signal. An antenna connected to the transmitter converts the electrical signals to electromagnetic waves, and an antenna connected to the receiver converts the electromagnetic waves back to electrical signals. Depending on the particulars of the communications modality, single or multiple antennas may be utilized.
Conventional transceivers typically do not generate sufficient power or have sufficient sensitivity for reliable communications standing alone. Thus, additional conditioning of the RF signal is necessary. The circuitry between the transceiver and the antenna that provide this functionality is referred to as the front end circuit, which is understood to be comprised of a power amplifier for increased transmission power, and/or a low noise amplifier for increased reception sensitivity. Each band or operating frequency of the communications system has a dedicated power amplifier and low noise amplifier tuned specifically to that operating frequency.
For a typical power amplifier utilized in WiFi applications, the gain requirement in the transmit mode is in the range of 25 dB to 30 dB. WiFi generally refers to multiple generations of local area networking standards designated as IEEE 802.11, each with different operating parameters. For instance, the maximum linear output power is approximately 18 dBm to 22 dBm in the 802.11g mode with an operating frequency of 2.5 GHz. The maximum linear output power in the 802.11a mode with an operating frequency of 5 GHz may be 17 dBm to 21 dBm. In light of these amplifier gain parameters and output power requirements, the transceiver output power is typically no more than −3 dBm.
However, in a calibration mode, the WiFi transceiver may increase the output power to as high as 10 dBm. The long-term reliability of the power amplifier may be compromised at these input power levels to the power amplifier, as there may be excessive voltage stress on the transistors thereof. The transistors in the last stages of the transmit chain are subject to the highest voltage stresses, as the previous stages amplify the input signal to levels high enough to cause damage. This is particularly problematic in CMOS (complementary metal oxide semiconductor) transistors that have lower breakdown voltage ratings compared to conventional BiCMOS or GaAs (gallium arsenide) technologies.
The transistors in the last amplifier stage are subject to additional stresses beyond that which is associated with the large input signal alone when the output is not perfectly matched to the 50 Ohm load. This may often be the case when the transceiver/front end circuitry is connected to automatic test equipment in a production line. This may result in the voltage level at the transistor terminals far exceeding that for reliable operation. Accordingly, there is a need in the art for improved architectures for protecting power amplifiers from input power overstress.